Radio base filters for filtering and combining of a number of sender units to one antenna cable are conventionally built up of various units, e.g. a cavity, a resonator, and one or more support units. The frequency range of such a filter is mainly determined by the dielectric properties of the resonator while the fine-adjustment of the filter frequency is achieved by means of adjusting the position of a tuner unit of an appropriate shape and consisting of a dielectric material in relation to said resonator unit. Commonly, this is achieved by means of inserting the tuner to a certain degree into a hollowness of the resonator. The tuning unit comprises, e.g., a ceramic tuner and a fastening means, e.g. a screw nut, that is movably arranged along a threaded axis in order to perform the movements of the tuner relative to the resonator. The threaded axis is driven by a motor unit whereby the screw nut transforms the radial movement of the threaded axis into a linear movement of the screw nut and the tuner.
For frequency tuning, e.g., in a radio base station, it is crucial to achieve a high precision for such frequency adjustments. This implies that the tolerances for undesired variations of the tuner position must be as small as possible. Preferably, a sliding screw or a similar arrangement is used.